PURPOSE: To enable presence of any trouble in a pressure reduction means, a closed circuit, a piping for connecting an air-liquid converter to the closed circuit, the air liquid converter, etc., to be known rapidly by a warning and such a dangerous state that a brake torque is not generated properly to be dissolved and also prevent the damage of a fluid type retarder following the leak of an operation liquid.

CONSTITUTION: A fluid type retarder device is provided with a fluid type retarder 11 which is mounted on a rotary shaft 10 rotating together with a wheel and in which an operation liquid is always filled, a closed circuit 21 for connecting the operation liquid entrance 11a of the fluid type retarder 11 to an operation liquid exit 11b, an air-liquid converter 25 for applying an air pressure to the operation liquid in the closed circuit 21 and pressure reduction means 32, 33, 34 for reducing the pressure of air supplied from a pressurized air source 4 to the air liquid converter 25. A pressure detection means 8 detects the pressure in the closed circuit 21 and judges whether the pressure in the closed circuit 21 is within a prescribed range or not and when it does not exist within the prescribed range, a warning is issued by a warning means 48.